<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>背景颜色</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      /* background-color: transparent;  透明的 */
      /* background-color: red; */
      background-color: green;
    }
  </style>

</head>

<body>
  <div></div>
</body>

</html>